1. Field of the Invention
The present invention relates to a reflector for use in a light emitting device, the reflector having a concave reflecting surface capable of reflecting light emitted from a plurality of directional light sources and of forming a uniformly irradiated surface, and also relates to a light emitting device using the reflector.
2. Description of the Background Art
As a light emitting device used for general illumination and a projector, a combination of a reflector having a concave reflecting surface and a discharge lamp is widely used.
However, the discharge lamp needs large power consumption and has large heat discharge. Thus, a light emitting diode (LED) has been proposed to be used as a light source of a light emitting device, since the LED needs less power consumption and has less heat discharge, and besides, an amount of light emission per LED is being increased in recent years. However, even if the amount of light emission is increased, the amount of light emission per one unit is still smaller than that of the discharge lamp, and in order to cover this disadvantage of the LED, a light emitting device having a plurality of LEDs is developed so as to emit a larger amount of light (for example, patent document 1: Japanese Laid-Open Patent Publication No. 2007-101732).
As shown in FIG. 14A, light emitting device 1 according to patent document 1 includes two LEDs 2, and a reflector 4 having a concave reflecting surface 3. The concave reflecting surface 3 has two half paraboloids 5 located side by side having a space therebetween. Each of the LEDs 2 is arranged at a focal point F of its corresponding half paraboloid 5, and emits light such that a light axis L thereof is oriented toward the center of the half paraboloid 5.
According to the light emitting device 1, light beams emitted from the respective LEDs 2 are reflected on the corresponding half paraboloids 5, and are outputted, as parallel light beams, from the light emitting device 1. Thus, when the two LEDs 2 are turned on simultaneously, the amount of light emission can be doubled.
In the light emitting device 1, it is possible to increase the amount of light emission as above described, however, on an irradiation target surface A, the same number of bright circular portions X as the LEDs 2 are formed by the parallel light beams from the half paraboloids 5, and a majority portion of the irradiation target surface A is covered with the bright circular portions X, and a dark portion Y is generated on the remaining portion. Accordingly, a difference between bright and dark portions, caused by a light distribution pattern, on the irradiation target surface A is increased, which leads to a problem since the irradiation target surface A cannot be irradiated uniformly.
This is because, as shown in FIG. 5, each LED 2 is a directional light source that emits light such that a light beam traveling on a light axis L (that is, a light distribution angle =0) has maximum intensity, and a light beam traveling at an wider angle relative to the light axis L has smaller intensity. As shown in FIG. 14, light L1, which travels on the light axis L, is reflected on the half paraboloid 5, and irradiates a point X1 (a point irradiated by light reflected at an intersection between the light axis L and the half paraboloid 5), has the maximum light intensity, whereas the light intensity is decreased when the light irradiates a point that is more distant from the point X1 on the irradiation target surface A.
In order to irradiate the irradiation target surface A uniformly, patent document 2 (Japanese Laid-Open Patent Publication No. 2006-73532) discloses a technique of convexly arranging, on a concave reflecting surface of a reflector, a large number of micro reflector segments each having a surface curved with a predetermined curvature radius toward an inner space of the reflector.
The reflector disclosed in patent document 2 uses, as a light source, a light emitting element such as a halogen lamp, which is obtained by spirally winding a filament so as to form a cylindrical shape. That is, a light emitting element having a certain length is arranged so as to protrude from a central portion of the reflector. Light having uniform intensity radiated from the halogen lamp toward the entire circumference and reflected on the concave reflecting surface. The light beams are reflected at certain angles and then diffused, respectively, on a large number of micro reflector segments which are arranged on the concave reflecting surface and are each curved with a predetermined curvature radius. As a result, the diffused light beams are mixed together, which increases a uniformity ratio of the light intensity on the irradiation target surface A.
However, even if the technique disclosed in patent document 2 is applied to the light emitting device 1, which includes a plurality of LEDs as the directional light source disclosed in patent document 1. It is not able to sufficiently improve the uniformity ratio of the light intensity on the irradiation target surface A.
This is because the curved radius of the micro reflector segments on the reflector disclosed in patent document 2 is fixed, and a degree of diffusion of the light caused by the micro reflector segments is uniform at any point on the concave reflecting surface. Accordingly, both of the light L1 traveling on the light axis L and having strong intensity and light having weak intensity (e.g., light traveling at an angle of 90 degrees relative to the light axis L) are diffused in a similar manner, resulting in creation of the “bright circular portion X” and the “dark portion Y” on the irradiation target surface A.